You can count on this, we have no plans of modifying this and if we somehow ended modifying az.plot_posterior is would have a deprecation warning for a very long time, which is what we are doing with az.plot_joint.
I do want to note that what you can’t count on being stable are the default values for many of the arguments in ArviZ functions, if you rely on az.compare using waic instead of loo you should either pass this explicitly to the function or use rcParams to set a default globally (most of the defaults that could change have an rcParam but not all of them do yet).
I think this will only be removed in 4.0, which is not far, check out the timeline to be up to date about this, things are a bit hectic right now. v3 should be frozen and still get bugfixes for a while but no new features will be added.